The invention relates to the field of coupled cavity structures, and in particular to an efficient harmonic generation and frequency conversion scheme in multi-mode cavity structures.
Nonlinear frequency conversion has been commonly realized in the context of waveguides, or even for free propagation in the nonlinear materials, in which light at one frequency co-propagates with the generated light at the harmonic frequency. A phase-matching condition between the two frequencies must be satisfied in this case in order to obtain efficient conversion. Moreover, as the input power is increased, the frequency conversion eventually saturates due to competition between up and down conversion. Previous experimental and theoretical work on second-harmonic generation in cavities has largely focused on cavities with a single resonant mode at the pump frequency. Such structures require much higher powers than our proposed doubly-resonant cavity, however, approaching one Watt and/or requiring amplification within the cavity.
Second-harmonic generation in a doubly resonant cavity, with a resonance at both the pump and harmonic frequency, have previously been analyzed only in the limit where nonlinear down-conversion can be neglected. Previous work on third-harmonic generation in cavities, similarly, considered only singly resonant cavities; moreover, past work focused on the case of χ(2) materials where 3ω is generated by cascading two nonlinear processes (harmonic generation and frequency summing). Furthermore, the previous theoretical work, with a few exception, focused on one-dimensional Fabry-Perot cavity geometries, in which the problem of obtaining cavity modes with the correct frequency ratio was posed as a problem of phase-matching, and addressed by methods such as using off-normal beams.